The SD20 requires version AE 1.0. Unfortunately, I don't think 2.5 will do (although it might be worth a try, if all else fails!). AE 1.0, and now AE1.5, are different from the previous versions, in that they allow editing of parts of clips, and not just complete clips.
The A.E 1.5 manual states that it will not work with AVCHD footage from pre 2008 cameras (i.e. SD1-5-9).
Whether the reverse is true, and modern camera footage will work with older HD Writer versions....?
There is an update on AE1.0 here:
HDWriter AE1.0 for HDC L052 Update Program | Digital AV Software Download | Digital AV Software | Digital AV | Consumer Products | Support | Panasonic Global,
but unlike the update for 2.0, it appears that you require the original software to be installed...again,might be worth a try!
As sjw suggests, the best option would be for Curry's to have another look for the original disc. If not, maybe they can get a replacement from Panasonic?